There has recently been proposed by Eastman Kodak a new organic EL element, particularly that of low molecular-weight type (OLED for short hereinafter), which realizes high luminance with a low driving voltage owing to its improved structure, with electrodes holding between them a charge transporting layer and an emitting layer formed by vapor deposition. (See Appl. Phys. Lett. 51 (1987) 913 and U.S. Pat. No. 4,356,429.) The ensuing active researches have led to the development of an element of three-layer type in which the emitting function is separated from the carrier transportation. Thus, the OLED element has entered the stage of practical use. (See Jpn. J. Appl. Phys. 27 (1988) L269, L713.) Moreover, a new EL element with a polymeric emitting material (PLED for short hereinafter) has been found in Cambridge College. (See Nature, 347 (1990), p. 539.) In addition, it has been found that the PLED has greatly improved characteristic properties if it is provided with a conductive organic material formed on the electrode. (See Appl. Phys. Lett. 64 (1994), p. 1245.) Now, such new PLED elements are comparable in characteristic properties to old OLED elements.
Conductive polymers are also finding use in the technology relating to capacitors. For example, a thin film of conductive polymer formed by electrolytic polymerization is gradually replacing manganese oxide interposed between the dielectric layer and the electrode for solution of problems with contact resistance.
Conductive polymers to be applied to such electronic devices are required to have a high solubility in solvents so that they provide a smooth thin film and infiltrate into minute gaps.
The present applicant invented a charge transporting varnish prepared from a low-molecular weight oligoaniline dissolved in an organic solvent and also found that this varnish gives rise to a hole injection layer which greatly improves the characteristic properties of EL elements. (See Japanese Patent Laid-open No. 2002-151272.)
Unfortunately, the aniline derivative has the quinoimine structure in its oxidized state, which remarkably impairs its solubility in solvents but contributes to its conductivity. For example, polyaniline polymerized by chemical oxidation has a very low solubility owing to its quinoimine structure. One way to increase solubility is to reduce the quinoimine structure with hydrazine. However, if a salt is formed by doping, the resulting product decreases in solubility. The problem with solubility remains unsolved.